Does increased chelation enhance the rate of ligand substitution at PtII (N,N,N) centres? A detailed kinetic & mechanistic study

Abstract

The rate of substitution of the chloride and aqua moieties from the platinum(II)-amine complexes, viz. [Pt(dien)Cl]Cl (Pt1-Chloro) and [Pt(en)(NH 3 )Cl]Cl (Pt2-Chloro) and their corresponding aqua analogues, viz. [Pt(dien)(OH 2 )] (ClO 4 ) 2 (Pt1-Aqua) and [Pt(en)(NH 3 )(OH 2 )](ClO 4 ) 2 (Pt2-Aqua), by a series of neutral and anionic nucleophiles,viz. thiourea (TU), 1,3-dimethyl-2-thiourea (DMTU), 1,1,3,3-tetramethyl-2-thiourea (TMTU), iodide (I ) ) and thiocyanate (SCN ) ), was determined under pseudo first-order conditions as a function of concentration and temperature using UV/Visible spectrophotometry and standard stopped-flow techniques. The observed pseudo first-order rate constants for the substitution reactions obeyed the simple rate law k obs =k 2 [Nucleophile]. Second-order kinetics and negative activation entropies, ca. )93 J K )1 mol )1 and )71 J K )1 mol )1 , for the chloro and aqua complexes respectively, support an associative mode of activation. The rate of substitution of both the chloro and aqua moieties are observed to decrease with an increase in the steric bulk of the neutral nucleophiles, whilst rate of substitution by SCN ) was observed to be faster than that of I ) , in correlation with the observed nucleophilicities of the two nucleophiles. A comparison of the second-order rate constants, k 2 , at 298 K, obtained for the substitution reactions of Pt1 and Pt2 shows that an increase in chelation in moving from Pt2 to Pt1 results in a corresponding increase in the reactivity, by a factor of ca. 3, (28.31±0.15 and 8.02±0.13 M )1 s )1 for Pt1 and Pt2 respectively, in the case of substitution of the aqua species by TU). Computational analysis of the chloro complexes, viz. Pt1-Chloro, Pt2-Chloro and [Pt(NH 3 ) 3 Cl]Cl (Pt3) support this conclusion by demonstrating that the Pt-N bond trans to the leaving group is shortened and that the Pt-Cl bond is lengthened when chelation is increased from Pt3 to Pt1. Consequently, these results suggest that the increase in reactivity of Pt1 over Pt2, promoted by increased chelation, is as a result of ground state destabilization.

A kinetic study of aqua ligand substitution in dinuclear Pt(II) complexes containing four non-coplanar pyridine ligands

“…Substitutions involving TMTU gave the lowest rates, whereas those with TU reacted significantly faster. This trend is in line with the size of TMTU nucleophile relative to TU and DMTU [39,46,47]. The activation parameters in Table 4 show that the entropies of activation are large and negative.…”

Substitution kinetics of platinum N,N,N’,N’-Tetrakis (2-pyridyl methyl) octane diamine aqua complex with thiourea, glutathione and L-cysteine nucleophiles

“…Therefore, kobs can be represented by the rate law given by kobs = k2 [Nu] where Nu = GLT, L-CYST Substitution reactions with TU, GLT, and L-CYST were performed at pH 2 where the complexes exist in their diaqua form, and protonation of the TU can be ruled out as reported by Elding et al [30] . The substitution reaction with TU is considerably faster than with GLT and L-CYST, and this is due to the fact that TU is a stronger nucleophile than GLT and L-CYST [8,23,33] . Also the order of reactivity of the nucleophiles (TU > L-CYST > GLT) can be linked with their bulkiness, i.e.…”

“…For this type of substitution reaction where there is an intercept, kobs can be represented by: kobs = k1 + k2 [TU] where k1 is given by the intercept of the plot of kobs vs [TU]. Jaganyi and Reddy [23] reported k2 values of 28.31 M -1 s -1 and 8.02 M -1 s -1 for the substitution of the aqua ligand by TU in mononuclear complexes [Pt(dien)(OH2)](ClO4)2 and [Pt(en)(NH3)(OH2)](ClO4)2, respectively, in 0.2 M NaClO4. In their studies, plots of kobs vs [TU] were linear without intercepts.…”

“…These studies are largely connected to understanding the mechanisms of their activities as homogeneous catalysts and anti-tumour drugs [20][21][22] . Desigan and Jaganyi [23] studied the substitution of the chloride and the aqua ligands from mononuclear platinum (ll) amine complexes, [Pt(dien)Cl]Cl, [Pt(en)NH3Cl]Cl, [Pt(dien)(OH2)](ClO4)2, and [Pt(en)(NH3) (OH2)] (ClO4)2 with a series of nucleophiles including TU, DMTU, TMTU, I -, and SCN -. While the rate of substitution decreased with increased steric bulk of the nucleophiles, it increased with increased chelation in the complexes.…”

Chelate ring closing and opening behavior in cyclopentadienyl cobalt(III) complexes with pendant nitrogen functional group

“…Although the interplay between electronic and steric properties has long been recognized as essential in determining the chemical or physical properties of a complex, predictions of reactivity can be very difficult, generally because of the small energy differences involved. Each metal may react differently towards the same ligand while different ligands can completely modify the chemistry of a given metal [5][6][7][8][9].…”